Gulls and Terns — Family Laridae 235

Bonaparte’s Gull Larus philadelphia The only small gull regular in San Diego County, Bonaparte’s Gull is most abundant on the ocean, where, in winter, it can often be seen in flocks of hundreds a short distance offshore. Along the coast, it is much less numerous, though still often com- mon in San Diego Bay or the north county lagoons. Inland, Bonaparte’s Gull is quite irregular, both from lake to lake and from year to year. Even along the coast Bonaparte’s Gull appears to be becoming patchier in its distribution, perhaps as a symptom of the faltering productivity of the ocean off southern California.

Winter: Offshore, the flocks of Bonaparte’s Gulls undoubt- edly shift widely with the shifting of fish and plankton. Photo by Anthony Mercieca Onshore, however, the species has a few favored con- centration points, especially the sewage ponds near the 100 on 14 December 1997, B. Schram). At other lakes our Santa Margarita River along Stuart Mesa Road (G5; up counts were of 15 or fewer. Inland, however, the species’ to 70 on 6 December 1998, R. E. Fischer), Buena Vista occurrence is quite irregular, as exemplified by the his- Lagoon (H5; 150 on 7 February 1999, J. Determan), Los tory of Lake Henshaw Christmas bird counts: Bonaparte’s Peñasquitos Lagoon (N7; 100 on 3 January 1999, D. K. Gull has been recorded on 17 of 22 counts 1981–2002, Adams), and the San Diego River mouth (R7; 200 on 22 with a median of 25, a mean of 59, and a maximum of 418 February 2000, V. P. Johnson). Bonaparte’s Gull is scarce on 23 December 1996. in Mission Bay but fairly common in San Diego Bay. The only winter records from the Anza–Borrego Desert On weekly surveys of central and south San Diego Bay are of birds lost far from water, along the Montezuma 1993–94, Manning (1995) noted a maximum of 138; on Grade of Highway S22 (G23) 2 January 1993 (E. Post) and weekly surveys in and near the salt works through the at Vallecito (M25) 12 December 2001 (S. Martin). same period, Stadtlander and Konecny (1994) noted a maximum of 108, though usually numbers were much Migration: Bonaparte’s Gull does not arrive in any lower. significant numbers until November. On their weekly Atlas observers also found Bonaparte’s Gulls on sev- surveys of north San Diego Bay 1993–94, Mock et al. eral inland lakes, especially Henshaw (G17; up to 185 on (1994), recorded an earliest date of 19 October, the earli- 7 February 1999, W. E. Haas), Sutherland (J16; 100 on est among any of the regular surveys of San Diego Bay. 22 February 2000, M. B. Stowe), and Hodges (K10/K11; One fall migrant has been noted in the Anza–Borrego Desert, on a golf-course pond at the Roadrunner Club (F24) 29 October–6 November 1991 (A. G. Morley). In spring migration Bonaparte’s Gulls make use of the migration corridor over San Diego County’s relatively low mountains, following the same route as the Brant, Surf Scoter, and Common Loon. During the atlas period our most strik- ing example of this was 300 at Lake Cuyamaca (M20) 2 April 2001 (P. D. Jorgensen). Spring counts at Lake Henshaw range as high as 490 on 31 March 1979 (R. Higson, AB 33:806, 1979). Another notable sighting was of eight flying west down Hauser Canyon (T21) 25 April 1998 (R. and S. L. Breisch). Dates for such migrants range from 27 March 236 Gulls and Terns — Family Laridae

(one at the Borrego sewage ponds, H25, A. G. Morley) to noticeably in the 1990s. The San Diego count, with a 50- perhaps 5 June (1998, nine at Lake Cuyamaca, A. P. and year average of 734, reached its all-time lows of four in T. E. Keenan). When spring migration ends is difficult to 1999, one in 2000, and three in 2002. The Rancho Santa say because of occasional summering birds in nonbreed- Fe count averaged 317 from 1980 to 1990 but 64 from ing plumage (Unitt 1984). We recorded no clearly sum- 1991 to 2003. The Oceanside count hit its post-1976 lows mering Bonaparte’s Gulls 1997–2001; one at San Elijo of three in 2000 and 53 in 2002, even though it hit its high Lagoon (L7) as late as 13 June 1998 (B. C. Moore) was still of 1400 in 2001. Similarly, the Escondido count found in breeding plumage. Bonaparte’s Gull regularly at Lake Wohlford (H12) 1987– 93 but only sporadically since. Such irregularity charac- Conservation: Numbers of Bonaparte’s Gulls on San terizes birds that feed on scarce, clumped resources. Diego County’s three coastal Christmas bird counts fell